Locality Group Affinity

The kernel assigns a thread to a locality group when the lightweight process (LWP) for that thread is created. That lgroup is called the thread's home lgroup. The kernel runs the thread on the CPUs in the thread's home lgroup and allocates memory from that lgroup whenever possible. If resources from the home lgroup are unavailable, the kernel allocates resources from other lgroups. When a thread has affinity for more than one lgroup, the operating system allocates resources from lgroups chosen in order of affinity strength. Lgroups can have one of three distinct affinity levels:

1. LGRP_AFF_STRONG indicates strong affinity. If this lgroup is the thread's home lgroup, the operating system avoids rehoming the thread to another lgroup if possible. Events such as dynamic reconfiguration, processor, offlining, processor binding, and processor set binding and manipulation might still result in thread rehoming.

2. LGRP_AFF_WEAK indicates weak affinity. If this lgroup is the thread's home lgroup, the operating system rehomes the thread if necessary for load balancing purposes.

3. LGRP_AFF_NONE indicates no affinity. If a thread has no affinity to any lgroup, the operating system assigns a home lgroup to the thread .

The operating system uses lgroup affinities as advice when allocating resources for a given thread. The advice is factored in with the other system constraints. Processor binding and processor sets do not change lgroup affinities, but might restrict the lgroups on which a thread can run.

Using lgrp_affinity_get()

The lgrp_affinity_get(3LGRP) function returns the affinity that a LWP has for a given lgroup.

#include <sys/lgrp_user.h>
lgrp_affinity_t lgrp_affinity_get(idtype_t idtype, id_t id, lgrp_id_t lgrp);

The idtype and id arguments specify the LWP that the lgrp_affinity_get() function examines. If the value of idtype is P_PID, the lgrp_affinity_get() function gets the lgroup affinity for one of the LWPs in the process whose process ID matches the value of the id argument. If the value of idtype is P_LWPID, the lgrp_affinity_get() function gets the lgroup affinity for the LWP of the current process whose LWP ID matches the value of the id argument. If the value of idtype is P_MYID, the lgrp_affinity_get() function gets the lgroup affinity for the current LWP.

The lgrp_affinity_get() function returns EINVAL when the given lgroup or ID type is not valid. The lgrp_affinity_get() function returns EPERM when the effective user of the calling process is not the superuser and the ID of the calling process does not match the real or effective user ID of one of the LWPs. The lgrp_affinity_get() function returns ESRCH when a given lgroup or LWP is not found.

Using lgrp_affinity_set()

The lgrp_affinity_set(3LGRP) function sets the affinity that a LWP or set of LWPs have for a given lgroup.

#include <sys/lgrp_user.h>
int lgrp_affinity_set(idtype_t idtype, id_t id, lgrp_id_t lgrp,
                      lgrp_affinity_t affinity);

The idtype and id arguments specify the LWP or set of LWPs the lgrp_affinity_set() function examines. If the value of idtype is P_PID, the lgrp_affinity_set() function sets the lgroup affinity for all of the LWPs in the process whose process ID matches the value of the id argument to the affinity level specified in the affinity argument. If the value of idtype is P_LWPID, the lgrp_affinity_set() function sets the lgroup affinity for the LWP of the current process whose LWP ID matches the value of the id argument to the affinity level specified in the affinity argument. If the value of idtype is P_MYID, the lgrp_affinity_set() function sets the lgroup affinity for the current LWP or process to the affinity level specified in the affinity argument.

The lgrp_affinity_set() function returns EINVAL when the given lgroup, affinity, or ID type is not valid. The lgrp_affinity_set() function returns EPERM when the effective user of the calling process is not the superuser and the ID of the calling process does not match the real or effective user ID of one of the LWPs. The lgrp_affinity_set() function returns ESRCH when a given lgroup or LWP is not found.